1. Field of the Invention
The present invention relates to a spark plug for an internal combustion engine.
2. Description of Related Art
A vehicle mounted-internal combustion engine is provided with spark plugs as an igniting means. The spark plug includes an insulator holding a center electrode inserted thereinto, a housing holding the distal end portion of the insulator inserted thereinto, and a ground electrode joined to the housing so as to from a spark discharge gap with the center electrode. The spark plug generates a discharge spark between the center electrode and the ground electrode by being applied with a high voltage from an ignition coil to destroy insulation of the spark discharge gap.
Japanese Patent Application Laid-open No. 2006-156110 describes such a spark plug. The spark plug described in this patent document has the structure in which the center electrode is constituted of a core member made of a highly thermal conductive material and a cover layer covering the distal end portion of the core member.
However, this spark plug has a problem as described below.
When the center electrode is heated, the core member expands with heat in the axial direction. At this time, since the proximal end portion of the center electrode is fixed, the distal end of the center electrode moves toward the ground electrode. As a result, since the spark discharge gap between the center electrode and the ground electrode becomes small, the igniting performance of the spark plug is degraded.